1. Field of the Invention
The present invention relates to the field of production packers for oil and gas mining, and more particularly, for coal bed natural gas production.
2. Description of the Related Art
Coal bed natural gas is an important facet of the nation's energy mix (1). Coal bed natural gas currently constitutes approximately seven percent of the nation's natural gas production, and it is expected to increase in importance as an energy source (1). Natural gas is clean-burning and can be used as a boiler fuel, vehicle fuel, and heating fuel (1). Coal bed natural gas is often produced at shallow depths with large volumes of water (1). Coal bed natural gas production can occur within and outside areas of conventional oil and gas production (1).
A production packer is a device that is used to isolate the annulus and anchor or secure the bottom of the production tubing string (2). The purposes of a typical production packer include isolating well fluids and pressures, separating producing zones to prevent fluid and pressure contamination, aiding in forming the annular volume required for gas lift or subsurface hydraulic pumping systems, limiting well control to the tubing at the surface, and holding well servicing fluids in the casing annulus (3).
Various production packer designs have been developed for use in different contexts, depending on the wellbore geometry and the production characteristics of the reservoir fluids (2). Most existing production packers are designed for use in connection with reservoirs that are at least 2000 feet deep. Sophisticated design elements, primarily involving the sealing element, have been developed to accommodate the higher pressures and temperatures that accumulate at such depths. Conventional packers are typically installed using cement or other materials pumped into an inflatable sealing element (4). Alternatively, the packer is set in place against the production casing using mechanical setting devices. In the latter instance, the sealing element is compressed either by set down weight or tension.
One of the problems with using conventional packers for production at shallower depths is that they are not cost-effective. Many of the features that have been developed for high-depth, high-pressure and high-temperature applications are simply not necessary in the shallower applications. Another problem with using conventional packers for coal bed natural gas production is that they do not address the problems associated with diminishing water levels above the coal seams. In coal bed natural gas production, the coal seams are dewatered to cause desorption of the gas molecules. Removal of the water, however, can lead to collapse of the coal seams due to the presence of fractures within the coal seams themselves.
Accordingly, it is an object of the present invention to provide a cost-effective production packer for use in connection with shallower reservoirs (typically in the range of 500 to 3000 feet). It is a further object of the present invention to provide a production packer design that allows multiple seams of coal to be dewatered and produced simultaneously. It is a further object of the present invention to maintain a constant head of water over each coal seam. These objects are accomplished by equipping the packer with separate production, gas vent and water overflow strings, as well as a submersible pump.